The following abbreviations are herewith defined, at least some of which are referred to within the following description of the prior art and the present invention.    CDMA Code-Division Multiple Access    FIR Finite Impulse Response    HSPA High-Speed Packet Access    PA Power Amplifier    PCM Pulse Code Modulation    PSK Phase Shift Keying    QAM Quadrature Amplitude Modulation    UMTS Universal Mobile Telecommunications Service    WCDMA Wideband Code-Division Multiple Access
It is well known in the art that a communications signal such as a radio signal can be thought of as having an instantaneous complex value on a two-dimensional complex plane, where the coordinate in one dimension is the signed amplitude of a sinusoidal component of the radio signal and the coordinate in the second dimension is the signed amplitude of a cosinusoidal component of the radio signal. Since the cosine function and the sine function are mutually orthogonal functions, their correlation is zero, which means that the two dimensions are at right angles with respect to each other. The two dimensions are commonly labeled I and Q for “In-phase” and “Quadrature”. No other orthogonal dimensions exist in this signal space, which is a plane. It is also well known that symbols representing groups of bits can be located on the I, Q plane such that any two symbols are sufficiently separated from one another to avoid confusion, so long as the signal-to-noise ratio is high enough. For example, 16 symbols can be located on a 4×4 grid known as 16QAM, and four binary bits assigned to each point. In 64QAM, 64 symbols can be located on a 8×8 grid and 6 binary bits assigned to each point. Non-rectangular constellations of points can also be used, for example 16-PSK (phase shift keying) in which 16 points are equispaced in angle around a circle, and 4 binary bits are allocated to each of the points.
In the prior art, it is generally considered to be desirable and known to allocate bit groups to symbol points according to a Grey coding scheme such that the bits assigned to adjacent points in the signal space differ in as few bit positions as possible, ideally in only one bit position. A brief discussion is provided next about some of these well known schemes and their drawbacks that are associated with the prior art.
U.S. Pat. No. 4,084,337, filed Aug. 24, 1976, describes a 4-dimensional modulation scheme in which both radio wave polarizations are used to provide two independent channels, where each channel is capable of carrying a two-dimensional signal. In this patent, there is reference to an IEEE paper entitled “Digital Transmission with Four Dimensional modulation” (Trans IEEE on Information Theory, July 1974, pp. 497-502) in which there is described a four dimensional modulation scheme that is constructed to have a peak energy constraint. The peak energy constraint was described therein to mean that the sum of the powers in the two polarizations should not exceed some maximum value. For instance, if (I1,Q1) are the In-phase and Quadrature Phase components on one polarization and (I2,Q2) are the In-phase and Quadrature Phase components on the other polarization, then the total energy or power which is constrained is given by I12+Q12+I22+Q22. This is a relevant and adequate constraint when (I1,Q1) and (I2,Q2) are separately generated and applied to physically independent channels, such as orthogonally polarized antennas. However, if (I1,Q1) and (I2,Q2) are not separately generated and not applied to physically separate channels, but instead are applied to the same physical channel, then the transmitted signal would be (I1+I2,Q1+Q2) and its energy or power would be proportional to (I1+I2)2+(Q1+Q2)2, which is not constrained by the same metric. Thus a different scheme is needed in order to constrain the peak energy in the latter case.
U.S. Pat. No. 4,597,090, filed Apr. 14, 1983, discloses a modulation scheme for a single physical channel in which the two dimensions in signal space (I,Q) on m sequential signal samples are considered to form a 2 m dimensional space, and where mN data bits are encoded into the 2 m dimensions in such a way as to obtain a coding gain by constraining the selection of I,Q value of one signal sample to depend on the selection of I,Q values for the other signal samples. This is a form of Trellis Coding, and is related to obtaining a coding gain but is silent about obtaining a reduction of the peak-to-rms ratio of the radio signal which is a subject that is related to the present discussion.
The 3rd Generation cellular system known as WCDMA or UMTS, currently has a method under specification known as HSPA which enables the transmission of higher data rates from a mobile phone to a network (or base station). The HSPA transmission uses an approach called “Multi-code CDMA”. In this Multi-code CDMA system, each data symbol is spread out in time and spectrum by combining it with a spreading code. On the downlink (base station to mobile phone), the codes used to transmit signals from the base station are coordinated at the base station so that they are mutually orthogonal. In contrast, in the uplink (mobile phone to base station), the coordination needed between different mobile phones to achieve orthogonality is considered too difficult to implement, so each mobile phone uses a different random code sequence.
However, at each mobile phone, it is still possible to generate several random code sequences that are coordinated among themselves to be mutually orthogonal. Each of these orthogonal codes may then carry a symbol sub-stream so that the combined symbol stream rate is enhanced. But, in this situation, the mobile phone's available transmitter power is going to be divided between the different codes which means that the range over which each sub-stream may be successfully received and decoded error free is going to be reduced. In fact, the reduction of power per each sub-stream in a multi-code modulation signal transmitted from a mobile phone is worse than would be expected by merely dividing the transmitter power by the total number of sub-streams. This is because it is not so much the average power that is constrained by battery voltage, but rather the peak signal amplitude, which happens to be limited by the battery voltage.
Thus, in the 3rd Generation cellular system there is a desire for a modulation scheme which develops the greatest mean power per each sub-stream within a constraint of the composite peak signal amplitude of all sub-streams. For example, if the mobile phone used a three-code multicode scheme with three length=4 codes where each code carried a sub-stream of 16QAM symbols at similar amplitude, then the total mean power that is transmitted within a given peak amplitude constraint is 7.32 dB below the peak before filtering to contain the spectrum, and the mean power per sub-stream is 12.1 dB below the peak. Filtering generally increases the peak-to-rms ratio further. The HSPA standard describes an improvement over the three length=4 spreading code scheme since it specifies a length=2 code of twice the power (√2 times the amplitude) which carries two symbols in the same time period that a superimposed and orthogonal length=4 code carries a third symbol, which effectively achieves the same symbol rate as three length=4 codes. This 4+(2,2) configuration is able to develop a total mean power which is 5.44 dB below the peak, and is 1.88 dB more effective than the 4+4+4 configuration that is associated with the three length=4 spreading code scheme.
However, with the 4+(2,2) configuration there is a reduction of the spreading factor, which is merely a move in the direction of no spreading which also achieves a low peak-to-rms ratio, but the resulting radio signal becomes very difficult to decode when there is a significant amount of multipath channel distortion. In view of the foregoing, it can be seen that there has been and is still a need for a transmitter and a method that can address the aforementioned shortcomings and other shortcomings associated with the prior art. These needs and other needs are addressed by the transmitter and the method of the present invention.